aller se coucher / aller dormir

Essaye "dort!" L'imperatif travaille bien

if it's an order, "aller" becomes "va" if singular (va te coucher !) or "allez" if you are talking to more than one personne or fi you want to sound more polite (allez vous coucher !)

morphée m'emporte. (with swag) or je vais dormir. (trivial)

you could also say aller au lit